SIMILARITIES AND DIFFERENCES IN LANCELET AND CRANIATE NERVOUS SYSTEMS

ABSTRACT

The central nervous system and some sense organs of lancelets show similarities to and differences from those of craniates. Lancelets and many craniates share such cellular elements in the spinal cord as the Rohde interneurons. Recent tracing experiments indicate that this similarity may extend to large paired cells in the hindbrain region, including the Müller cells and perhaps the Mauthner cells. Similarity between lancelets and craniates may end at the hindbrain/midbrain boundary, which is possibly demarcated by the expression of the Engrailed gene. While most craniates develop some ocular motoneurons adjacent to this area of gene expression, lancelets appear to lack such motoneurons. Topological similarity in this region of the neuraxis apparently does not extend to exact replication of cellular fate in the two groups. Similarities in the central projections of primary sensory cells of lancelets and of the placodal cranial ganglia in craniates are discussed in relation to their possible homology and to the potential role of neurotrophins in their development. The recently discovered autofluorescent cells at the anterior end of the lancelet neuraxis lack homologues in craniates and may represent an autapomorphic character acquired by lancelets but not inherited by the craniates.